Get it right on the tiles


If you are looking to specify a tiled floor, it is crucial to understand what preparation is required to ensure that new or existing floor substrates are suitable. Stuart Ross of BAL looks at the different likely substrates, and the steps that need to be taken before tiling

Floating floors are typically those not attached to a rigid substrate, so for example a tongue and groove or chipboard floor on top of a layer of insulation or acoustic material.

Where this is the case, excessive movement in the subfloor is common and tiling directly to this type of subfloor without additional measures will likely result in the floor failing. As stated in BS 5385: Part 3, clause Floating floor: “Note 2: Direct fixing of ceramic tiles to a wood floating floor entails a high element of risk and where practicable should be avoided.” This is because the wooden floor is not supported in the same way as it would be by joists. So, support for the floor is limited.

Some floating floors often include heating pipes as well, which presents further challenges for rigid floor finishes especially where boarded systems are concerned. It is worth noting here however that floating timber boarded systems for rigid tile and stone finishes should be avoided in areas where water-fed heating pipes are installed, as this can cause excessive movement leading to tile failures.

However, where the floor is deemed to be suitable to be tiled, one potential solution is to improve rigidity on non-heated floating floors by over-boarding the floor with

10-12 mm tile backer board or an additional 15 mm WBP grade plywood – sealed with an acrylic primer or SBR sealer.

Boards should be installed staggered, so they do not coincide with any joints in the existing timber layer and leave a slight gap between the boards to allow for expansion. Use a tile adhesive as a bed below the board and screw the board at 300 mm centres, in accordance with British Standards. It is important that installers do not use nails, as movement in the floor can cause them to work free and cause problems down the line.

Even if deflection is taken out of the floor sufficiently, we would recommend tiling with a flexible and highly deformable S2 tile adhesive, as these have extra polymers to cope with any movement. Please note that even if you overlay the floor, it may still be possible that there is too much deflection in the floor, in which case the floor is unsuitable for receiving ceramic tiles.

We would recommend contacting your adhesive manufacturer, who can provide onsite support to assist as well as further installation guidance.

Timber floors
As with floating floors, timber floors should be structurally stable for tiling and be capable of carrying both static and dynamic loads without excessive deflection.

This may mean the need to fit additional noggings between joists and replace any damaged or broken planks.

We would recommend overboarding the timber with either a 12 mm tile backer board or a minimum 15 mm thick plywood of suitable quality, glued and screwed at 300 mm centres ensuring screw heads are flush with the surface.

If the floor needs to be levelled, then consideration should be given to using a suitable fibre reinforced levelling and smoothing compound. You must first prime the surface of the timber with two coats of neat acrylic primer, allowing it to dry between each coat and best practice to apply the second coat at 90° to the first.

When tiling direct over timber we would recommend installing an uncoupling mat system. Uncoupling mats provide a “buffer” layer between the timber floor and tiled surface which helps prevent any lateral movement from causing any damage to the tiles.

A suitable highly polymer modified flexible tile adhesive and grout should be specified.

Concrete / sand & cement screeds
Before tiling to concrete or sand and cement screeds, it is essential that the floor is clean and free from any barriers to adhesion, sound, dry and fully cured before tiling. New concrete must be cured and allowed to dry in air for at least six weeks before tiling, while new sand and cement screeds need a minimum of three weeks.

If you’re tiling onto a screed that incorporates underfloor heating, there are more considerations before fixing can commence. This includes the proper commissioning of the heated screed before and after tiling. After a minimum three week drying out period for the heated screed, water fed systems should be gradually heated at a rate of 5°C per day from the minimum operating temperature up to a maximum operating water temperature of 45°C as recommended by the heating manufacturer. The heating should be maintained at this level for a minimum of three days before cooling to room temperature. In colder temperatures, maintain the floor at 15°C before tiling.

If heated screed isn’t gradually ‘commissioned’ before tiling commences, movement stresses exerted can damage the screed and the newly applied tiling layer. This is caused by a combination of significant drying shrinkage of the screed occurring, along with additional thermal expansion/contraction.

Electric cable systems keep the heating system off for five days to allow for levellers and adhesives to cure, then gradually turn up by no more than 5°C per day up to a temperature not exceeding 27°C. The heating system should be gradually increased after tiling i.e. 14 days after grouting for normal setting tile adhesives.

Calcium sulphate screeds
Calcium sulphate floors are a common floor substrate nowadays, particularly in new builds, however they require specific preparation before they are suitable for tiling over.

Under ideal drying conditions, these screeds take approximately 1 mm per day up to 40 mm to dry, and an additional two days per mm for screeds greater than 40 mm, e.g. 60 days for a 50 mm thick screed. However, this can often extend during colder periods of the year and higher atmospheric humidity (e.g. above 65 per cent RH). It is important that all barriers to adhesion should be removed after two to six days. Moisture levels should be less than 75 per cent relative humidity or a residual moisture content of lower than 0.5 per cent water by weight when measured using a carbide moisture tester.

Priming is also key before tiling commences on calcium sulphate screeds, as this creates an effective barrier between the gypsum and cement-based products – protecting against an adverse reaction. The primer should also be adequately absorbed into the screed surface, following the manufacturer’s advice.

If a fast-track system is needed, there are systems on the market including floating uncoupling systems and DPMs which can reduce the time required before tiling commences.

Tiling to the correct type of steel can be achieved, if it is rigid and free from deflection and prepared properly. This might involve a suitable mechanical preparation to remove any surface coatings and to expose a shiny keyed steel surface.

To provide a suitable mechanical key to receive a tile adhesive and tile one option is to use a suitable epoxy coating which is sand-blinded as a priming coat. This provides a good key prior to the application of a suitable cement-based tile adhesives or a specialist S2 rubber-crumb tile adhesive.

Be mindful that if the floor being tiled is likely to be subjected to constant wetting i.e. a wet room or shower/bathroom, then the subfloor should be waterproofed with an appropriate tanking solution. This is to protect the floors from water ingress and prevent porous materials being adversely affected. This is required because tiles, cement-based adhesive and grouts are not inherently waterproof.

Stuart Ross is technical services manager at BAL